The present invention relates to a method and a system for increasing the heat collection efficiency of solar ponds and for reducing operating cost.
The known solar pond is essentially a black-bottomed basin, usually 100-200 cm. deep, filled with saline water, in which a density gradient is maintained to prevent convection. Israel Pat. No. 12,561 of May 25, 1959 achieves the purpose of maintaining said required density gradient by addition of solid solute or concentrated solution to the bottom region of the pond and addition of fresh water or dilute solution to the surface region of the pond and draining some liquid from the surface region of the pond.
After 15 years of research and development efforts, one of the inventors, Dr. Harry Tabor has stated that "a collection efficiency of 20% and an operating temperature around 90.degree. C. leading to a turbine efficiency of 10%, i.e. 2% overall efficiency is expected " (see "The National Symposium of Energy" held on Mar. 19-21, 1974, Netania, Israel, page 377--published by "State of Israel, the Ministry of Commerce and Industry, National Resources Research Administration" Jerusalem, September 1974).
The reason of this low collection efficiency (20%) is the requirement in said known method to maintain a dilute solution at the surface of the pond as a main means to achieve and maintain a vertical density gradient. The direct result is that the bulk of the solar radiation penetrating the surface of the pond is lost to the atmosphere by evaporation from said dilute solution, due to its relatively high vapor pressure.
In the following table are given the yearly evaporations at the Dead Sea region for solutions of increasing densities, and the yearly evaporation from the carnallite producing basins, for densities of 1,300 and 1,335:
______________________________________ Density Yearly evaporation/m.sup.2 ______________________________________ 1.175 1600 kg 1.200 1520 kg 1.210 1420 kg 1.217 1330 kg 1.300 600 kg 1.335 455 kg ______________________________________
These data are given on pages 8,9 and 12 of a report by "Dead Sea Committee--1955/1956 sub-Committee on Evaporation" published 1956, Jerusalem.
Now, the daily evaporation from the Dead Sea surface (density=1.175) is 4.4 kg/m.sup.2 compared to the daily evaporation of 1.65 kg/m.sup.2 from the adjacent carnellite basins (or ponds) at a density of 1.3.
The corresponding heat of evaporation is 2550 koal/m.sup.2 and 950 koal/m.sup.2 respectively.
The difference of 1600 keal is 32% of the total incident radiation of 5000 koal/m.sup.2 per day, as generally assumed.
According to FIG. 3 in "The Physics of the Solar Pond" by H. Weinberger, page 46 of "Solar Energy" Vol. 8, No.2, 1964, and according to Table I in "Solar Pond Project" by H. Tabor and R. Mats, page 180 of "Solar Energy" Vol.9, No.4, 1965, more than 40% or more than 2000 koal/m.sup.2 per day of the solar radiation reaches the depth of 50 cm. and penetrates into the liquid body below this depth.
The new solar pond method utilizes a highly concentrated acqueous solution, having a density of 1.3 approximately at the surface, and enables to trap in a new manner and to utilize the heat penetrating the depth of 50 cm. (or even less) of the liquid body. Thus more than 40% of the incident radiation or more than 2000 koal/m.sup.2 per day are utilized compared to a collection efficiency of less than 1000 koal/m.sup.2 per day as expected by the inventors of the known solar pond methods.
In U.S. Pat. No. 3,647,638 of Mar. 7, 1972 there is described a "Feed-Liquid-Lift System" and there is discussed the "Clementine" method of lift. Also in U.S. Pat. No. 3,741,878 of June 26, 1973 the principles of the above mentioned U.S. Patent are used and some improvements are added.
Now, the object of these methods of lift is to obviate the need of a pump for each evaporator or at each stage of a multi-stage desalination plant based on the known vapor-reheat flash evaporation process.
Between any two consecutive evaporators or stages, which are hermetically insulated against the atmospheric pressure, there is always a positive pressure difference which assists the flow of the liquid feed from the stage of higher pressure to the stage of lower pressure, through a connecting U-tube, or a "well type manometer" system.
The present invention deals with a different case, where there is only one flushing stage (or series) or evaporation chamber (or series). This flushing stage or evaporation chamber is provided with a downward liquid discharge column having a downward extension submerged in the pond open to atmospheric pressure and with an upward liquid feed column connected to a liquid feed supply container (also) open to the atmosphere, the surface level of said latter container being lower than the surface level of the pond.